An important attribute of paperboard used as packaging material is its stiffness. High bending stiffness may be desirable in many paperboard packaging applications. For example, bending stiffness is important in folding box container paperboards because the utility of the box depends upon its resistance to bulging when filled. The higher the bending stiffness of the paperboard, the more rigid will be the box made from that paperboard, and thus the greater the resistance of that box against loading or crushing forces.
The ability of paperboard to resist bending during the manufacture of packaging, such as boxes, may also be important. Paperboard is often bent in converting and packaging machines. The paperboard may be bent over rolls of such machines, and thus formed to certain curvatures. If, for example, the roll diameter of the packaging machine is small, the paperboard may be highly curved, thus imparting high tensile stresses on the convex side of the paperboard, and high compression stresses on the concave side. If these stresses become too high, the paperboard may be damaged by fractures and wrinkles on the surfaces thereof, as well as reducing the bending stiffness of the paperboard.
In addition, fold-crack resistance may be important for paperboards (untreated or treated with, for example, pigment coatings, fluorochemical treatments for grease resistance, moisture barrier coatings, heat sealing coatings, extrusion coatings, etc.) which are folded to form the packaging by influencing the functionality and appearance of the packaging made from the paperboard. Cracking at the fold of the packaging may lead to strength reduction, as well as the appearance of a visible crack at the folded surface of the packaging. In fact, bending stiffness and fold cracking are related, with a higher bending stiffness leading to a higher risk of fold cracking.